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Natural causes
The Earth’s climate can be affected by natural factors that are external to the climate system, such as changes in volcanic activity, solar output, and the Earth’s orbit around the Sun.
Of these, the two factors relevant on timescales of contemporary climate change are changes in volcanic activity and changes in solar radiation. In terms of the Earth’s energy balance, these factors primarily influence the amount of incoming energy.
Volcanic eruptions are episodic and have relatively short-term effects on climate. Changes in solar irradiance have contributed to climate trends over the past century, but since the industrial revolution, the effect of additions of greenhouse gases to the atmosphere has been over 50 times than that of changes in the Sun’s output.
Human causes
Climate change can also be caused by human activities, such as the burning of fossil fuels, deforestation and the conversion of land for agriculture and for setting up factories.
Since the beginning of the industrial revolution, these human influences on the climate system have increased substantially. In addition to other environmental impacts, these activities change the land surface and emit various substances to the atmosphere. These, in turn, can influence both the amount of incoming energy and the amount of outgoing energy and can have both warming and cooling effects on the climate.
The dominant product of fossil fuel combustion is carbon dioxide, a greenhouse gas. The overall effect of human activities since the industrial revolution has been a warming effect, driven primarily by emissions of carbon dioxide and enhanced by emissions of other greenhouse gases.
The build-up of greenhouse gases in the atmosphere has led to an enhancement of the natural greenhouse effect. It is this human-induced enhancement of the greenhouse effect that is of concern, because ongoing emissions of greenhouse gases have the potential to warm the planet to levels that have never been experienced in the history of human civilization. Such climate change could have far-reaching and/or unpredictable environmental, social, and economic consequences.
Earth’s temperature is a balancing act
Earth’s temperature depends on the balance between energy entering and leaving the planet’s system. When incoming energy from the Sun is absorbed by the Earth system, Earth warms. When the Sun’s energy is reflected back into space, Earth avoids warming. When absorbed energy is released back into space, Earth cools.
Many factors, both natural and human, can cause changes in Earth’s energy balance, including:
- Variations in the Sun’s energy reaching Earth
- Changes in the reflectivity of Earth’s atmosphere and surface
- Changes in the greenhouse effect, which affects the amount of heat retained by Earth’s atmosphere
These factors are the main causes of climate change, and have caused Earth’s climate to change many times for thousands of years.
Greenhouse-effect causes atmosphere to retain heat
When sunlight reaches Earth’s surface, it can either be reflected back into space or absorbed by Earth. Once absorbed, the planet releases some of the energy back into the atmosphere as heat (also called infrared radiation). Greenhouse gases like water vapor (H2O), carbon dioxide (CO2), and methane (CH4) absorb energy, slowing or preventing the loss of heat to space.
In this way, GHGs act like a blanket, making Earth warmer than it would otherwise be. This process is commonly known as the “greenhouse effect.” Changes in reflectivity affect how much energy enters Earth’s system.
When sunlight reaches Earth, it can be reflected or absorbed. The amount that is reflected or absorbed depends on Earth’s surface and atmosphere. Light-colored objects and surfaces, like snow and clouds, tend to reflect most sunlight, while darker objects and surfaces, like the ocean, forests, or soil, tend to absorb more sunlight.
References
USGCRP (2014). Climate Change Impacts in the United States: The Third National Climate Assessment. [Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds.] U.S. Global Change Research Program.
IPCC (2013). Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
NRC (2010). Advancing the Science of Climate Changes . National Research Council. The National Academies Press, Washington, DC, USA.
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